This application is based on patent application No. 10-338018 filed in Japan, the contents of which are hereby incorporated by references.
This invention relates to a light measuring apparatus for measuring a light amount of visible radiation, infrared radiation or ultraviolet radiation such as a light meter, an infrared intensity meter, an ultraviolet intensity meter, a light power meter, a color meter or a luminance meter, and a light measuring method, particularly to zero-point calibration (offset correction) of a light measuring apparatus.
A light measuring apparatus is mainly comprised of a photodetector for converting a received light into a current and then outputting it, and a circuit for converting the current outputted from the photodetector into a voltage and calculating a light reception amount based on the voltage value or for integrating the current outputted from the photodetector for a predetermined period and calculating the light reception n amount based on this integral value. As a method for the zero calibration (zero-point correction) of the light measuring apparatus are known a method which is carried out by shading the photodetector as, for example, disclosed in U.S. Pat. No. 4,201,472 and a method which is carried out by electrically disconnecting the photodetector and the calculating circuit as, for example, disclosed in U.S. Pat. No. 4,924,081.
According to the calibration method carried out by shading the photodetector, a state where no light is incident on the photodetector is set as a zero reference of the measuring apparatus. Thus, this method should be treated as a standard in guaranteeing a measurement precision. However, this method has a disadvantage of cumbersome calibration since calibration has to be manually performed because of a necessity to shade the light receiving portion of the apparatus by a special cap or the like. In order to improve operability of this calibration method (hereinafter, xe2x80x9cstandard zero-point calibration methodxe2x80x9d), it may be considered to provide the apparatus main body with a barrier or shutter for shading the photodetector. However, such a construction has not been put into practice due to limits in an arrangement space, complication of construction, the size of the apparatus and a production cost.
On the other hand, according to the method carried out by electrically disconnecting the photodetector and the calculating circuit from each other, a switch S1 is provided between a photodetector SP1 and an amplifying circuit AMP (constructing a part of the calculating circuit) for amplifying a light current outputted from the photodetector SP1, a switch S2 is provided in parallel with the photodetector SP1 as shown in FIG. 8. A state where no light is incident on the photodetector SP1 is realized by a circuit by turning the switch S1 off while turning the switch S2 on (by disconnecting the photodetector SP1 from the amplifying circuit AMP). Thus, an offset amount for the zero-point calibration is calculated without shading the photodetector SP1.
This calibration method enables automation of the zero-point calibration and reduces cumbersomeness of the manual operation in the standard zero-point calibration method. However, since the zero reference is located not on a light receiving surface of the photodetector SP1, but at an input end of the amplifying circuit AMP, this is, strictly speaking, different from the standard zero-point calibration method. This method is put into practice as a method for compensating the standard zero-point calibration method since a difference between the offset amount calculated according to this method (hereinafter, simple zero-point calibration method) and the one calculated according to the standard zero-point calibration method is so small as to cause no problem in practical level.
Normally, when the light measuring apparatus is started, zero-point calibration is first performed. Thereafter, light measurement, i.e., measurement of illuminance, light intensity, or luminance is performed without performing zero-point calibration unless particularly required.
However, the offset amount used for the zero-point calibration may change during measurement. For example, in the case that temperature in the working environment of the apparatus changes, the offset amount of the calculating circuit for the zero-point calibration may change by being influenced by the temperature change. Further, in the construction for realizing the simple zero-point calibration by electrically disconnecting the photodetector SP1 and the amplifying circuit AMP by means of an electronic switch such as a transistor, it is impossible to completely electrically disconnect them, and a part of the light current outputted from the photodetector SP1 leaks into the amplifying circuit AMP. Accordingly, the offset amount for the zero-point calibration changes also in the case that the leak current changes due to a change of the electronic switch with time. Furthermore, the above offset amount may change due to a change in the leak current from a logic signal for controlling the electronic switch. In a measuring apparatus having a plurality of measurement ranges to ensure a sufficiently large total measurement range. The above offset amount may vary by switching the measurement ranges.
If the offset amount for the zero-point calibration changes during the measurement, it is not preferable since the reliability of measurement values is considerably reduced even if measurements of high precision are possible. In the light measuring apparatus for performing a zero-point calibration only according to the standard the zero-point calibration method, an operator has to manually shade the photodetector for the zero-point calibration in a zero-point calibration mode. Thus, the reliability of the measuring apparatus depends on handling of the operator. Therefore, it is difficult to securely guarantee the reliability of the apparatus.
On the other hand, U.S. Pat. No. 4,924,081 discloses only the simple zero-point calibration method, but contains no disclosure on a method for solving the above problem using the simple zero-point calibration method. Neither does this patent disclose any method for securely performing the zero-point calibration by combining the standard and simple zero-point calibration methods to stabilize the measurement precision of the light measuring apparatus.
It is an object of the invention to provide a light measuring apparatus and a light measuring method which have overcome the problems residing in the prior art. According to an aspect of the invention, a light measuring apparatus comprises a photoelectric conversion element for converting a light energy to an electrical signal, a calculator for calculating a measurement value based on an electrical signal, a memory for storing a correction value, a corrector for correcting the measurement value using the correction value stored in the memory.
There is further provided a timer for measuring an elapse of time after the correction value is stored in the memory, and a controller for allowing calculation of a recent correction value when the timer measures elapse of a predetermined time, and permitting the memory to renewably store the recent correction value.
Alternatively, there may be provided a controller responsive to a measurement range change for allowing calculation of a recent correction value each time the measurement range is changed, and permitting the memory to renewably store the recent correction value.
According to another aspect of the invention, a method for measuring light using a light measurement apparatus provided with a photoelectric conversion element for receiving and converting a light energy into an electrical signal, and a calculation circuit for calculating a measurement value based on the signal. The method comprises the steps of storing a first correction value for correcting a measurement value, judging whether a predetermined time elapses after the first correction value is stored, calculating a second correction value when the predetermined time is judged to elapse, renewably storing the second correction value, and correcting the measurement value with the second correction value.
Alternatively, the method comprises the steps of storing a first correction value for correcting a measurement value, detecting whether the measurement range is changed, calculating a second correction value when the measurement range is changed, renewably storing the second correction value, and correcting the measurement value with the second correction value.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.